In a digital synchronous network for transmitting a data signal multiplexed as a frame signal such as a digital synchronous transmission system including a synchronous digital hierarchy (SDH), synchronous optical network (SONET) or optical transport network (OTN), a large phase fluctuation (jitter or wander), if any, of the data signal would make it impossible to transmit the data signal correctly.
In constructing a digital synchronous data transmission system, therefore, it is necessary to accurately measure the resistance to the phase fluctuation and the transmission characteristic of the various transmission devices of the system in advance.
This measurement requires accurate determination of the phase variation of the data signal applied to the transmission devices to be measured and the phase variation of the data signal output from the transmission devices.
As a conventional technique for grasping the phase variation of the data signals, a method is known wherein the data signal is input to the oscilloscope and the width W of the level transition portion of the eye pattern displayed as shown in FIG. 7 is observed (Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 5-145582).
The phase variation is of either random noise type or dependent on the pattern of the data signal. In the observation of the eye pattern described above, the user can grasp only the maximum value of the phase variation as a combination of both types, and cannot determine the phase lead or lag of each bit position of the frame signal such as the actual data signal composed of predetermined bits.